There are two basic types of nuclear imaging systems for medical usage. Gamma imaging involves imaging one emission photon at a time, and collimators are usually part of the design. Positron imaging involves two emissions photons emitted in almost opposite directions, and collimators are not required as long as the detection system detects both photons. The invention of combined magnetic resonance imaging systems and nuclear medicine imaging systems began with U.S. Pat. No. 4,939,464 filed in 1989 by Hammer, which disclosed a combination NMR/PET scanner which uses light pipes to communicate the scintillation events to the exterior of the magnet. In this design, the detector could not be placed in the area of large magnetic fields because the materials and design used within the detector were adversely affected by magnetic fields, so light pipes connected the scintillator's optical output signals to the detectors which were outside the magnetic field area. The concept of using light guides and fiber optic connections between scintillator and detector have continued in other system designs, including U.S. Pat. No. 5,325,855 filed in 1992 in which the fibers offered flexibility of positioning for a surgeon, and U.S. Pat. No. 5,719,400 filed in 1995 and U.S. Pat. No. 7,835,782 filed in 2006 which uses optical fiber to allow positioning of the detectors outside of the magnetic field of the MRI system. An alternative design approach uses MR compatible PET detector systems that allow the detectors to be placed within the MR bore, and which then use detector output connection methods to connect the detector output to the outside world. This has been done in “MR Compatible PET Using Tileable GAPD Arrays”, J H Jung et al, IEEE 2009 Nuclear Science Symposium Record, M13-27, pp. 3556-3559. These MR-PET hybrid systems are designed to allow simultaneous imaging to occur. Simultaneous imaging is achieved when the same region of space is imaged by both imaging methods at the same time. In order to achieve simultaneous imaging, the two imaging systems must be in imaging position at the same time, and must be designed to be compatible with each other.
PET systems are built for detecting two annihilation photons which are emitted in nearly opposite directions, with both photons being at an energy level of 511 keV. PET systems use two detecting heads at opposite sides of the volume under study, and they use electronic collimation instead of physical collimation. PET systems are designed to detect only 511 keV photons. Gamma detection systems, on the other hand, usually use a physical collimator because the energy is emitted only in a single direction. A collimator is built of a slab of heavy metal, typically lead, into which is drilled or fashioned a pattern of holes. The gamma rays can be emitted at energy levels ranging from 81 to 365 keV, depending on which compounds are injected into the patient. One common gamma camera front end design consists of collimator, scintillator, detector, and electronics, with the collimator excluding all gamma rays except those that line up with the openings in the collimator, the scintillator converting the gamma photon into an optical signal, the detector converting the optical signal to an electrical signal and the electronics conditioning that signal to allow further processing or display functions to occur. There has always been concern that introducing the metal collimator into the MRI bore during imaging would lead to imaging artifacts or other MRI image degradation. If the physical collimator is positioned between the RF coil and the patient, then the RF coil will not be able to image. Alternatively, if the RF coil is placed between the patient and the collimator, one must ensure that the RF coil does not alter the path or energy of the gamma photon. Positron emissions have a higher energy, and so are less affected by intervening materials. U.S. Pat. No. 7,394,254 discusses this issue, and provides an RF coil invention that is more transmissive of nuclear radiation.
Hybrid MR-Gamma imaging has been discussed within U.S. Pat. No. 7,629,586 which describes a ring-based gamma camera concept that is axially oriented with the bore of a superconducting MRI system. In this patent, the RF coil is closest to the patient, outside of which is the gamma camera, outside of which is then the gradient coil. The gradient coil is typically built into the bore of the MRI system. Usually, the RF Coil will be a receive only coil, with the transmit portion of the MRI imaging function being performed by the transmit coil which is built into the bore of the MRI system. In this invention, rotation of the camera about the bore axis is discussed. This design geometry is very close to the PET-MR design geometry, in which the superconducting bore axis is also the axis for a ring-based nuclear imaging camera. In this type of design, the collimator is kept away from the RF coil so that interference does not occur, and the distance from the patient to the camera is quite large. For human imaging, a typical MRI bore will be 70 cm in size, a typical gamma camera depth will be nominally 6 cm, and so the effective bore size in such a design is approximately 58 cm because the gamma camera in this design moves from one area of the bore to the other, thus requiring 6 cm to be reserved from both top and bottom of the bore. For existing smaller bore superconducting magnets of 60 cm nominal bore size, this gamma camera design leads to 48 cm effective bore size, which is restrictive for some bariatric patients. As well, for specific types of imaging positions and usages, such as human prone breast imaging, a hybrid MR-Gamma design such as this would have a gamma camera quite far from the breast, leading to a reduced sensitivity and accuracy. This type of design approach requires an RF coil which is gamma compatible. Additional work on a movable axially-oriented MR-Gamma hybrid system has been shown in “A Prototype of the MRI-Compatible Ultra-High Resolution SPECT for in Vivo Mice Brain Imaging”, J-W Tan, L. Cai and L-J Meng, 2009 IEEE Nuclear Science Symposium Conference Record, pp. 2800-2805. In this paper, the SPECT system is moved in and out of the bore of the MRI on a non-magnetic gantry. Rotation about the bore axis is possible. In this design, the gamma camera is also outside the MRI RF receive coil. For an application such as human prone breast imaging, this design also suffers from having a gamma camera which may be quite distant from the breast area. As well, the RF coil that lies between the gamma camera and the patient needs to be gamma compatible. A thesis from London Ontario's Western University, by James William Kristian Odegaard (2007) entitled “Design and Performance Evaluation of a Small-Animal Pinhole-SPECT Array Insert for Field-Cycled MRI” discussed the organization of a SPECT camera as an insert into a field cycled MRI system. This insert is oriented along the axis of the MRI, and is not movable. This design also requires an RF coil that is gamma compatible. Previous work by Goetz et al [“SPECT Low-Field MRI System for Small Animal Imaging”, C. Goetz et al, J. Nuc. Med Vol. 49 (1) January 2008 pp 88-93] has also shown non-simultaneous imaging in which a slab magnet is used. This paper discussed a bore aligned gamma camera which allows a small animal to be moved from the gamma camera area to the MRI imaging area along a common axis. It does not allow movement of the gamma camera into the magnetic field. The gamma camera and MRI bore are aligned.
Additional work on the development of fixed RF coil and gamma camera systems includes the designs of S. Ha et al, as shown in “Development of a new RF coil and γ-ray radiation shielding assembly for improved MR image quality in SPECT/MRI “Phys Med Biol. 2010 May 7; 55(9):2495-504. Epub 2010 Apr. 6. In this case, holes were provided in the packaging of the RF coil, a specialized collimator mixture was used to form MR compatible collimator material that was inserted into these holes, and the gamma camera was positioned behind the holes and some distance from the RF coil.
Additional system design work is discussed in “Development of an MR-compatible SPECT system (MRSPECT) for simultaneous data acquisition”, Mark J Hamamura, Seunghoon Ha, Werner W Roeck, L Tugan Muftuler, Douglas J Wagenaar, Dirk Meier, Bradley E Patt and Orhan Nalcioglu, Published 17 Feb. 2010, Phys. Med. Biol. 55 (2010) 1563-1575. As the title indicates, this design is for simultaneous imaging, which requires that both gamma and MR system be in imaging position at the same time, which therefore requires that MR and gamma compatibility is required of the various system elements. Most importantly, in this design the RF coil is of a birdcage variety, and the collimator is moved directly through the rungs of the birdcage coil. The sample can be rotated to allow SPECT imaging. In this case, the effect of the collimator on the MR imaging is shown to cause changes to RF coil loading, and so adjustment of the coil trim capacitors is required. This design only allows for insertion of the collimator through the rungs, and so the depth of the collimator must be sufficient to extend from the back side of the RF coil packaging to the imaging position that the application requires. In this design, the collimator is inserted directly through the rungs of the birdcage coil, and so the separation of the rungs dictates the width of the collimator. The sample to be imaged, however, might be larger than the width of the rungs. For example, for breast or brain imaging in which an RF birdcage coil is used, the specific area to be imaged may be of larger size than the width between the rungs. In the case of breast cancer, the breast is typically of size 11 cm width with a pendant length of 10 to 15 cm, so the birdcage rung width would need to be very large to accommodate so large a collimator. Commercially available birdcage coils do not have such large rung spacings. Also, the area to be imaged may not be directly behind the area outlined by the rungs, and so the sample must be rotated to allow the desired area to be imaged more closely. For human breast or brain imaging, and indeed for many human and animal imaging situations, including diagnostic, interventional and intra-operative imaging applications, it is not possible nor permissible to rotate the patient. In human medical imaging applications it may also not be possible to rotate the coil. For example, for brain imaging during brain surgery, the lower part of the head coil is usually fixed in place throughout the operation, and so rotation is not allowed. Importantly, for this design to operate in simultaneous imaging mode, they discuss the alteration of the RF Coil trim capacitors based on amount of collimator insertion that occurs. Altering RF Coil trim capacitors is not allowed on most commercially available RF coils, and so this type of design may be required to have a customized coil design.
An additional application of interest is US 20100264918 invented by Roeck and Nalcioglu in which is disclosed a unique motor design for rotating a specimen that can be simultaneously imaged by SPECT and MRI methods. They use the same figures for RF birdcage coil and collimator orientation as are used in the paper above, and are authors of the above paper as well. In this design, the animal being imaged is rotated about the bore axis of the magnet. They indicate that they can improve post-processing of the SPECT image using MRI data, however they do not discuss changing the position or orientation of their collimator based on MRI data. In this invention, there is no concept of altering the orientation of the collimator, but there is the concept of moving the collimator closer to the sample or further away from the sample. This design also uses a collimator which is the same width as the width between the rungs of the birdcage. This design also uses a custom designed bird-cage coil. This invention does not discuss the alteration of the capacitors based on the depth of insertion of the collimator, but we assume that simultaneous imaging and optimal operation of the RF coil would require such a capacitor adjustment.
To summarize, the previous work has shown a few MR gamma hybrid system designs that are focused on simultaneous imaging of the sample, with these systems not being optimized for some medical applications such as human breast imaging in the prone position using commercially available RF coils. These existing designs require gamma compatible or specialized RF coils, and are not designed to interwork with existing commercially available RF breast coils. The one non-simultaneous hybrid system moved the sample between the MRI and nuclear imaging positions.
There are various designs for RF coils. A typical head-imaging coil uses a birdcage design which has openings to allow access and visibility. This type of coil design for the head is provided by various companies, with a particular focus on allowing sufficient room between the coil and head to allow other instruments to be introduced if intra-operative and interventional applications need to be performed. As well, it is necessary to allow visibility for the patient in those cases where an awake patient is being imaged.
For breast imaging similar types of coil designs have been discussed within US Patent Application 2009/0118611. In this design, a butterfly tape RF coil design is suggested which will cause the inner surface of the RF coil to be some distance from the breast being imaged. Also for breast imaging, coils may be built into the upper body surface or lower table surface to allow for ease of access for breast biopsy, therapy, ablation or needle and marker placement.
For brain and breast imaging and for imaging other body parts that have an RF coil some distance from the body, if hybrid MR-gamma imaging is desired, it would be useful to have an imaging method, system and device that allows the gamma camera to be positioned close to the patient. Allowing the gamma camera to image close to the patient will improve imaging specificity, sensitivity, reduce patient dosage levels and improve spatial resolution of the imaging. It will also allow more flexibility in the materials used for RF coil and gamma camera, leading to increased product availability and lower cost. It may also allow the gamma camera to be used in retrofit fashion with existing RF coils. The previous designs are limited because the collimator width is the same as the birdcage rung width. A different insertion method might allow better imaging for some applications. It may also be useful in some applications to have a gamma camera that can be inserted and removed from the patient area to allow optimum patient access for follow-on procedures such as biopsy, ablation, therapy and needle or guide insertion if necessary for interventional and intra-operative procedures. The previous designs have not indicated any method whereby they could be removed for interventional tool or device access. It may also be useful to have a removable gamma camera so that sterilization of the gamma camera packaging is not required. For example, some procedures and workflows would have a surgeon accessing the breast or brain area through the MRI coil using surgical instruments, and so if the gamma camera remains in place it would need to have more stringent sterilization procedures than a design that did not cause the gamma camera to remain in place. It may also be useful to have a gamma camera that can take on different orientations or spatial distances depending on the size of the body part, such as a breast, that is being imaged. Breasts vary in size from one patient to the next, and the suspected tumor location may change the optimum position for the gamma camera. As well, for brain surgical interventions the head may be positioned differently within the RF coil. As well, for small gamma cameras that are used for lymph node imaging, there may be restricted access to the breast tail and underarm area. It would also be useful to have a gamma camera that can be moved within the RF coil volume so that different types of RF coils, or different sizes of RF coils, may be used with a single gamma camera design. It may also be useful to have a movable gamma camera because different radioisotopes may be best imaged from different directions or distances. It would also be useful to have a gamma camera that can be inserted and removed from the RF coil so that the gamma camera can also serve in situations where MR systems are not used. For example, it is possible that a patient cannot be imaged in the MRI system due to claustrophobia issues or because the patient has metal items inside the body, but that a scintimammography session would still be useful for the patient, and so with a removable gamma camera system it is possible to also use the gamma camera for non-MRI based situations and applications, including breast screening, breast diagnostic imaging, breast biopsy imaging and guidance, bone scintigraphy, breast neo-adjuvant therapy monitoring, and other uses that are known in the art. If the RF coil and gamma camera are built together or fixed together in some way, then additional and multiple applications may not be possible. If the coil and camera systems are built together in some way, then replacement of equipment elements may become more complex and more costly. In addition, it would be useful to have a gamma camera architecture and design that is useful for both superconducting bore MRI systems as well as slab systems, for both vertical field and horizontal field applications.
The geometries and designs described herein offer improved usages for some medical imaging applications.